。
She brags about her car.
Literal
She [topic-は] her own car [object-を] is-boasting-about.
X を 自慢する frames an everyday object of pride. With ~ている, 自慢している reads either as habitual ('has the habit of bragging about her car') or ongoing ('is bragging about it now'). Cars sit squarely in the everyday: common pride-of-ownership items, often functioning as expressions of taste, status, or hard-earned savings.